Ususally it takes honorable active duty service during wartime in an area relating to said war. It is also issued to fresh airman, soldiers, marines, seaman etc. when joining during wartime. Also anyone who is guard or reserve and becomes eligible for an Armed Forces Expeditionary Medal, Vietnam Service Medal, or Southwest Asia Service Medal is automatically qualified for the NDSM.
Hope this clears things up
http://www.gruntsmilitary.com/natdef.shtml